Field of the Invention
The present invention relates to a touch screen, a touch panel including the touch screen, a display device and an electronic device including the touch panel.
Description of the Background Art
A touch panel is a device for detecting a touch with an indicator such as a finger and determining the position coordinates of the touched position on the touch panel, and has been attracting attention as one of excellent user interfaces. Nowadays, various types of touch panels have been commercialized, such as resistive touch panels, capacitive touch panels and the like. Generally, a touch panel includes a touch screen incorporating a touch sensor (a sensor for detecting a touch), and a detecting device that determines the position coordinates of the touched position based on a signal input from the touch screen.
As one type of the capacitive touch panels, projected capacitive touch panels are known (for example, Japanese Patent Application Laid-Open No. 2012-103761). A projected capacitive touch panel as disclosed in Japanese Patent Application Laid-Open No. 2012-103761 can detect a touch even when a glass plate, which has a thickness of about several millimeters, covers the front surface of a touch screen incorporating a touch sensor. Such a projected capacitive touch panel is capable of including a protective plate on the front surface of a touch screen, and thus excels in durability. In addition, even when a user wearing gloves touches a screen, a projected capacitive touch panel can detect a touch. Moreover, without movable portions, a projected capacitive touch panel has a long service life, since potential problems caused by movable portions can be avoided.
A projected capacitive touch panel includes, for example, a first series of conductive elements formed on a thin dielectric film and a second series of conductive elements formed above the first series of conductive elements through an insulation film as detection wirings for detecting a capacitance. The first and second series of conductive elements form a plurality of intersections without electrically contacting with each other.
According to a structure disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. H09-511086 (1997), a detection circuit detects a capacitance formed between an indicator, such as a finger, and the first series of conductive elements and second series of conductive elements, which are detection wirings, to determine the position coordinates of a position touched by the indicator. Such a method of detecting the position coordinates is generally called a self-capacitance system.
For example, a change in a mutual capacitance, specifically, a change in an electric field between a plurality of row wirings extending in a row direction, which form first electrodes, and a plurality of column wirings extending in a column direction, which form second wirings is detected to determine the position coordinates of the touched position (see Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2003-526831, for example). Such a detection method is generally called a mutual-capacitance system.
In either of the above self-capacitance system and mutual-capacitance system, a method is generally employed in which when a planar region (detection cell) divided into a grid pattern by the row wirings and the column wirings is touched by an indicator such as a finger, the position coordinates of the touched position are determined based on a balance between a detection value of the touched detection cell (sensor block) and a detection value of a detection cell near the sensor block.
Sensor capacitors are generally formed by row-direction wirings and column-direction wirings. It is ideal that sensor capacitors are formed such that electrostatic capacitance values of all pairs of sensor capacitors are equal to each other in a state in which no physical quantity acts on the sensor capacitors. However, when a touch panel is used in combination with a display module, such as a liquid crystal display (LCD), a parasitic capacitance is excessively formed between lead lines outside the touch screen and a display module such as an LCD, and thus an offset occurs in an electrostatic capacitance of the sensor capacitors even in a state in which no physical quantity acts on the touch screen.
Specifically, when a touch panel is mounted on an LCD having a front frame, capacitive coupling between the outermost one of lead lines, which surround the outer periphery of a detection area, and the front frame becomes larger than capacitive coupling between the other lead lines and the front frame, and thus an offset occurs in an electrostatic capacitance of the sensor capacitors. As a result, a dynamic range of a controller (a circuit that performs detection of a touch capacitance of each detection wiring, and the like) commonly connected to the lead lines is entirely reduced, and thus detection sensitivity of the detection wiring connected to the outermost lead line having a capacitance larger than the other lead lines may be degraded or disabled.
It is hard to distinguish between an offset of the electrostatic capacitance described above and a difference of the electrostatic capacitance caused by an action of a physical quantity in the output voltage of a projected capacitive touch screen, and this causes a detection error of a physical quantity.